Project Summary/Abstract The human skin immune system permits survival of several strains of bacteria that perform important functions to assist the skin immune defense system in defense against pathogens and regulation of inflammation. The goal of this proposal is to define factors that can promote or inhibit the survival of the beneficial members of the human skin microbiome. In particular, we will focus on molecules that influence survival of some strains of coagulase-negative staphylococci (CoNS) that produce antimicrobial molecules (AMs) (CoNS-AM+). These beneficial microbes compete with S. aureus for survival on the epidermis. Our prior work from laboratory based studies, animal studies and clinical trials with these bacteria have shown that CoNS-AM+ bacteria are an important part of the combined immune defense strategy of the skin, and their presence on skin will preclude colonization by S. aureus. This is particularly important in the skin disease atopic dermatitis (AD). In AD we have observed that CoNS-AM+ are deficient and replaced by strains of CoNS that lack AM activity (CoNS-AM-). Compelling preliminary evidence has now identified specific molecules produced by the epidermis and the core microbiome that shape the capacity of other microbes to inhabit this environmental niche. Our central hypothesis is that the production of these molecules dictates the composition of the skin microbiome. To test this hypothesis we propose 3 focused and inter-related specific aims. In the first aim will we induce divergent inflammatory responses in the skin, study the expression of the molecules that we hypothesize will select for survival of CoNS-AM+ over CoNS-AM- strains, and examine cytokines that regulate this process. In Aim 2 we focus attention on the microbes themselves, determining how the host factors interact with the microbe. Finally, in Aim 3 we introduce the important variable of the harmful microbe and how it also interacts with this system to attempt to outcompete CoNS-AM+ beneficial bacterial. The proposed project will bring better understanding of the pathogenesis of AD, and define mechanisms why AD skin lacks bacterial strains that can improve disease outcome. These data will be important in developing new and innovative therapies for AD and other inflammatory skin disorders.